1. Field of the Invention
The present invention relates to an exhaust timing control system and control unit. More particularly, the present invention relates to an exhaust timing control system and control unit of a snowmobile.
2. Description of Related Art
Snowmobiles are powered by internal combustion engines, particularly two stroke engines. Additionally, small snowmobiles do not have batteries to power electrical accessories when the engine is not operating; such accessories are powered by a generator when the snowmobile engine has been started. Larger snowmobiles may have batteries; however, due to the unusually cold operating environments that these vehicles are subjected to, the batteries may store and retain little power.
The conventional two stroke engines which power snowmobiles generally include an exhaust port provided in each cylinder wall such that spent gases are exhausted through the exhaust port as the piston reciprocates in the cylinder. Exhaust port timing has an important effect on engine performance. Optimum exhaust port timing is dependent, in part, upon engine speed. For instance, to provide an improved engine performance, the exhaust port timing can be advanced during high-speed engine operation relative to the exhaust port timing during engine idling.
One manner of controlling the exhaust port timing is to employ exhaust control valves. Generally, these valves are of the sliding or rotating type, and do not serve to ever completely close the opening or port in each combustion chamber. Instead, each valve moves between a first position, in which the valve does not obstruct, or obstructs very little of, the exhaust port, and a second position, in which the valve partially obstructs the port. Therefore, the exhaust control valve can alter the effective cross-sectional area of the exhaust port by appearing to lower an upper surface of the exhaust port, thereby restricting the flow through the exhaust port.
Because these valves have a relatively small range of movement, and are not continuously moved, they may seize within the guide passage in which they are mounted. Such seizure may result due to deposits that form on the valve bodies. Specifically, because these valves are normally employed in two cycle engines and because oil is typically mixed with the air and fuel charge ported into the combustion chamber, the oil can cause a further problem once the engine is shut off. While the engine is running, the engine temperature will be high enough to avoid any carbonization of the oil byproducts on the valves. However, as a stopped engine cools, residual oil may carbonize, or coke, on the control valves and form deposits which, along with other foreign matter, will make it difficult to operate the valves when the engine is later restarted. In short, these deposits will tend to inhibit smooth operation of the valves. Of course, the aforementioned temperature differences will also further aggravate this situation.
In the past, it has been suggested to exercise the exhaust control valve through a number of cycles between an opened and a closed position to clean any scale, carbon deposits or other debris from the exhaust control valve. In vehicles having sufficient battery power, the control valves are cycled through a cleaning operation under power from the battery before the engine is started. However, in a vehicle such as a snowmobile, which does not have a battery, a cleaning operation may not occur prior to starting.
Additionally, movement of the exhaust control valves typically alters engine performance characteristics such that efficiencies may be obtained at a lower engine speed range without significantly harming engine performance at a higher engine speed range. For instance, closing the exhaust control valves delays the timing of the exhaust cycle such that the compression ratio is increased. This increase enhances low speed engine performance. Again, due to the cold environment in which snowmobiles are used, the cold starting temperature of the engine typically causes difficulties when starting the engine. Moreover, the cold temperature can result in rough idling and rough engine running at low speed.